For a piezoelectric ink jet head that uses the electrostrictive effect of a piezoelectric element as the drive power source and is employed in an on-demand type ink jet printer, one having such a constitution is widely employed that comprises a plurality of pressure chambers to be filled with an ink disposed on one side of a plate-shaped substrate along the surface, with a nozzle for discharging the ink provided to communicate with each of the pressure chambers and a drive section including the piezoelectric element provided for each of the pressure chambers, as described in Japanese Unexamined Patent Publication JP-H-05-318731-A2 (1993).
In the piezoelectric ink jet head described above, a drive voltage is individually applied to one or more of the piezoelectric elements each corresponding to each of the pressure chambers so as to deform, thereby decreasing the volume of the pressure chamber that corresponds to the piezoelectric element, so that the ink contained in the pressure chamber is discharged from the nozzle that communicates therewith in the form of ink droplet and a dot is formed on a sheet of paper.
More specifically, a drive section comprising the piezoelectric element and an oscillator plate that supports the piezoelectric element transmits a force generated by the piezoelectric element as a pressure to the ink contained in the pressure chamber, thereby to function as a drive power source that discharges ink droplets through the nozzles that communicate with the pressure chambers. That is, the drive section causes the piezoelectric element to deform due to the drive voltage applied thereto, so that the oscillator plate is caused to deflect and protrude toward the pressure chamber, thereby decreasing the volume of the pressure chamber and pressurizing the ink in the pressure chamber, so that an ink droplet is discharged from the tip of the nozzle.
At the same time, since the oscillator plate is caused by the pressure of the ink contained in the pressure chamber to deflect in a direction opposite to that described above, the drive section also acts as an elastic body with respect to the vibration of the ink in the head.
When a drive voltage is applied to the piezoelectric element so as to generate a force, the ink contained in the head undergoes vibration under the pressure transmitted via the oscillator plate from the drive section. This vibration is generated as the drive section and the pressure chamber act as the elasticity against the inertia of a feeder port that feeds the ink to the pressure chamber, a nozzle passage that communicates with the pressure chamber and the nozzle, and the nozzle. Natural period of vibration of volumetric velocity of the ink contained in the head during this vibration is determined by the dimensions of the components described above, physical properties of the ink and dimensions and physical properties of the drive section.
In the piezoelectric ink jet head, an ink droplet is discharged by utilizing the vibration of ink meniscus in the nozzle due to the vibration of the ink described above, thereby forming a dot on the paper surface.
In order to achieve a higher resolution of the piezoelectric ink jet head while decreasing the size of the piezoelectric ink jet head, pitch of arranging the nozzles must be made as small as possible. When resolution of the piezoelectric ink jet head becomes higher and the number of nozzles increases, however, it becomes difficult to dispose the independent piezoelectric elements individually in correspondence to the pressure chambers. For this reason, it has recently become a prevailing practice to employ a piezoelectric ink jet head having a piezoelectric element made in a thin plate of transverse vibration mode that is formed integrally with an electrode (common electrode), lower (oscillator plate side) one of a pair of electrodes that are disposed to sandwich the piezoelectric element for applying the drive voltage to the piezoelectric element, and the oscillator plate, in such a size that covers the plurality of pressure chambers (hereinafter referred to as a “common element type”). Of the pair of electrodes, the electrode that is disposed over the piezoelectric element (individual electrode) is separately formed in a predetermined shape that corresponds to each pressure chamber for applying drive voltage individually to each piezoelectric element.
In the piezoelectric ink jet head of common element type, when an electric field is generated by applying the drive voltage from the individual electrode to the region sandwiched by the individual electrode and the common electrode in the plane of the piezoelectric element (hereafter referred to as a “drive region”), the drive region can be driven like an independent piezoelectric element thereby pressurizing the ink in the corresponding pressure chamber.
In order to individually apply drive voltages to the individual electrodes and ground the common electrode in the piezoelectric ink jet head of common element type described above, electrical connection must be established for the electrodes by soldering leads or using crimp terminals.
As taught in Japanese Unexamined Patent Publication JP-H11-34323-A2 (1999), making electrical connection within the area of the pressure chamber leads to a problem of variability in vibration characteristic of the piezoelectric element among the drive regions, due to the variability in rigidity and/or weight of the solder in the case of soldering, and due to the variability in crimping force in the case of using crimp terminals. For this reason, Japanese Unexamined Patent Publication JP-H11-34323-A2 proposes to make electrical connection of the electrodes outside of the area of the pressure chamber.
The present inventors studied the possibility of improving the vibration characteristic of individual drive regions of the piezoelectric ink jet head of common element type, and improving the vibration characteristic of the individual piezoelectric elements of the conventional piezoelectric ink jet head having the piezoelectric elements separately formed for the individual pressure chambers (hereafter referred to as a “separate element type”), by making the oscillator plate thinner than that of the prior art, specifically setting the ratio t1/t2 of the thickness of the piezoelectric element t1 to thickness of the oscillator plate t2 in a range from 1/1 to 1/4.
In such a piezoelectric ink jet head having thinner oscillator plate, however, it was found that it is not sufficient to make electrical connection of the electrodes outside of the area of the pressure chamber, and the possibility of vibration characteristic differing among the vibration regions or among the individual piezoelectric elements remains.